This invention relates to improved apparatus and processes for the manufacture of semiconductor wafers by means of a plasma etch and, in particular, to etching sloped contacts and vias in dielectric layers.
In the prior art, a variety of techniques have been proposed or even used for etching access holes, vias, in dielectric layers to connect conductive layers at different levels or to open contact areas to devices formed on a wafer. One technique comprises coating the wafer with photoresist, transfering a pattern into the photoresist, heating the photoresist so that it softens, thereby rounding the corners formed and encroaching slightly into the bottom of the opening formed previously, then etching both the photoresist and the underlying dielectric layer. The taper of the aperture formed in the dielectric depends upon the relative etch rates of the dielectric and the photoresist and on the slope of the photoresist at the start of the etch, among other factors.
A problem with this technique is the change in the dimension of the aperture in the photoresist. One has difficulty ensuring that the effect of the bake is always the same so that the resulting geometry of the opening in the dielectric is always the same. Inconsistent results are known as "loss of critical dimension." This problem becomes particularly acute as device geometries become smaller.
Another technique is to perform a multi-step, repetitive switching of chemistries to etch the dielectric slightly, then the photoresist, then the dielectric, then the photoresist, etc. The result is a stairstep profile which approximates the desired taper. As might be expected, reproducibility becomes a significant problem, as is dimensional control. A similar technique, described in U.S. Pat. No. 4,764,245, varies the composition of the gas mixture to change the selectivity of the etch.
Another technique, described in U.S. Pat. No. 4,764,249, requires placing an aperture mask some distance from the wafer and anisotropically etching the dielectric through the mask. While having the advantage of not requiring process changes, one has substituted equipment changes for the process changes, with the resultant problem of reproducibility. Further, one has a problem assuring registration of the mask with the appropriate portions of the wafer, particularly at small geometries.
In view of the foregoing, it is therefore an object of the present invention to provide an improved process for forming vias having a predetermined slope.
Another object of the present invention is to provide a process for forming vias having a predetermined slope without chemical or mechanical changes.